Production wise, in the days of physical models, it was easier to film them that way. Once CG became a thing, a matter of convenience, probably.Why do ships always meet the same way up?
I do like it when other sci-fi shows play around with this trope. For example, in Stargate when they were fighting the Ori, everyone tries to position their ships either in front of or behind the Ori ships. The Ori ships break formation and attack from above and below, which no one is prepared for.
Then sometimes, it's taken too far, like in Star Wars Episode 3 when General Grievous's flagship is taken out it practically "sinks."